I changed the symbol position to "After amount" and changed the Symbol to €test just to make sure that its exactly this record, and it is, but the currency field is still displayed before the amount.

currency_id = fields.Many2one(
        "res.currency", default=lambda self: self.env.company.currency_id, readonly=True
    )

    lohnkosten_m = fields.Monetary(
        currency_field="currency_id"
    )

view:

 <group>
                            <field name="currency_id" invisible="1"/>
                            <field name="lohnkosten_m" widget="monetary" options="{'currency_field': 'currency_id'}" />
</group>

Odoo v17